Damaged ligaments, cartilage and tendons in joints are not an uncommon occurrence, particularly with today's emphasis on physical activity and conditioning. One of the joints which requires particular skill and presents particular difficulties in repairing is the knee joint.
Numerous improvements in repairing damage to knee joints have been made over the years, and some of the major advances involve endoscopic techniques and arthroscopic procedures. Arthroscopic surgery is particularly useful in excising or repairing damaged knee cartilage.
Endoscopic techniques have also been developed for use in repair and reconstruction of damaged anterior cruciate ligaments (ACL) and posterior cruciate ligaments (PCL). When the ACL in particular has ruptured and is nonrepairable, it is usually replaced in young adults and the knee reconstructed through use of grafts (biological or synthetic).
The function of the real cruciate ligaments is complicated. The ACL and PCL are three-dimensional structures with broad attachments and a continuum of fibers. These fibers are of different lengths, have different attachment sites, and are under different tensions. Although many current substitutes for cruciate ligaments have not duplicated the complex orientation and operation of normal ACLs, they operate the best and mimic the normal ACL operation the best when they are placed isometrically. “Isometrically” positioned means that the length of the substitute ligament will not change during annular movement of the tibia relative to the femur; the distance between the affixed ends of the ligament remains a constant. Isometric placement maximizes the number of fibers that can be taut throughout the range of motion of the knee and allows for early knee motion without generating high ligament strains.
Correct isometric positioning of the ACL graft thus is an important factor for a successful operation. Correct isometric placement reproduces corresponding femoral and tibial anatomic attachment sites and will allow an ACL graft to mimic the normal ACL. Isometrically placed grafts provide the most stable knees. Non-isometric graft placement can result in plastic deformation of the ACL substitute, postoperative laxity, abnormal kinematics, or failure of fixation.
The importance of accurate placement of an ACL substitute is shown by the fact that graft placements sometimes only several millimeters apart produce significantly different strains in the cruciate substitute. A placement of the ACL origin or insertion which is too anteriorly placed in the knee joint results in a ligament that is taut in flexion, but lax in extension. Posterior placement causes the ligament to be taut in extension, but lax in flexion. Only isometric placement provides stability throughout the range of motion.